Delayed blowback/gas unlock action?

As an offshoot of my earlier Modern carbine in... .30 carbine? (http://www.thehighroad.org/showthread.php?t=349407) and Dream carbine...part 2 (http://www.thehighroad.org/showthread.php?t=350539) threads, I wonder if anyone knows of a gun using a simple, delayed-blowback action which uses a gas piston to unlock the breech?

Presumably, if gas tapped near the muzzle is used to unlock the breech, then the bullet has almost or maybe already left the barrel by the time the breech opens and pressures would be substantially reduced, allowing more powerful cartridges or a lighter bolt compared to straight blowback actions.

To put it another way, say a rotating bolt is spring-loaded to rotate and lock on closing, the gun fires, the bolt remains locked in the breech, the bullet nears the muzzle, the gas piston kicks the bolt against the spring pressure to unlock it, and the bolt is then free to recoil as a normal blowback action now that pressures have dropped to safe levels.

You have described a locked breech action, not a retarded blowback or delayed blowback.

cluttonfred

March 27, 2008, 08:20 AM

Thanks for the clarification, I'm certainly no expert. I guess I was thinking that if the breech was locked at the moment of firing, but that it was still the blown back casing that cycled the bolt and action, it would be considered a form of delayed blowback. As I understand it, most locked breech weapons involve a barrel that recoils with the bolt (even if for a tiny distance) to impart the momentum to cycle the action before things unlock.

BigG

March 27, 2008, 08:25 AM

There are actually two forms of locked breech autoloaders. Recoil operated and gas operated which was your first idea. You described recoil in your second post.

Delayed blowback means the breech is free to move as soon as the bullet moves. Various means are used to inhibit the free recoil of the breech, thus allowing a lighter breechblock than might be necessary with a blow back action.

Jim Watson

March 27, 2008, 09:13 AM

I have not heard of such a gun. If you are going to build a gas system (or recoil movement) then you had just as well let the machinery do all the work instead of letting it unlock so fast as to allow blowback of the expended case and bolt.

I had thought the Japanese Type 11 and Type 92 might qualify, but W.H.B. Smith said they were just knockoffs of the gas operated Hotchkiss guns. They had to have oilers because the Japanese did not make a faithful copy and their guns did not have any primary extraction. There was no (or not enough) residual chamber pressure to do any work.

There was a SIG rifle that was roller locked but gas operated. All work was done by the bolt carrier moved by the gas piston. SIG had been in cahoots with H&K but they went their separate ways because SIG wanted positive locking instead of H&K delayed blowback.

SDC

March 27, 2008, 09:19 AM

Something sort of similar was used in an early prototype of the Garand and in some of the SPIW flechette weapons, but instead of gas, they used the impulse of the primer backing out of the primer pocket to unlock the bolt. It's a system that can be made to work, but it's finicky and it depends on consistent ammunition to work properly.

SaMx

March 27, 2008, 11:26 AM

what about the gas delayed blowback action the P7 uses? I'm not sure exactly how it works but it seems like it would basically be self adjusting for lighter or heavier loads.

Vern Humphrey

March 27, 2008, 11:34 AM

There are penalties to using blow-back actions in rifle-caliber weapons -- one of them is the necessity of either lubricating the cases or fluting the chambers. The German G3, which is a roller-delayed blowback, uses the latter method.

Badger Arms

March 27, 2008, 11:53 AM

As others have said, you are getting two separate and unique actions comingled. There HAVE been firearms that used both gas and recoil action. The only system I'm currently aware of is the 'muzzle booster' device. See how it operates here: http://en.wikipedia.org/wiki/Muzzle_booster

cluttonfred

March 27, 2008, 11:57 AM

I have read somewhere that the H&K P7 pistols were pretty finicky about ammunition loading (not a problem in military or police use where ammo is standardized).

Again, to clarify, regardless of what it's called, I am talking about a fixed barrel and a recoiling bolt propelled by the rearward movement of the just-fired cartridge case, just like a straight blowback design, except using some sort of lock (say a rotating bolt with lugs) tripped by gas pressure to delay the recoil until pressures have come down to a safe level.

Jim Watson

March 27, 2008, 12:05 PM

What about it?
It is strictly delayed blowback with no mechanical locking at any time.

I always admired the primer setback action that SDC mentions. The early Garands used it but when they changed from the old fast burning Pyro DG powder to more progressive IMRs, it did not work well enough with the more gradual buildup of chamber pressure.

There was once another primer setback action that used a case with a thick head and a primer at the bottom of a very deep pocket. The primer and that deep primer pocket served as a piston to drive the strong firing pin back to run the action. Maybe that was the AAI SPIW or maybe it was Something Else, I don't recall.

BigG

March 28, 2008, 02:29 PM

Whichever way you decide to build it, the key feature is building enough delay time for the bullet to leave the bbl and pressure to drop. If you open the breech with the bullet still in you will have a Ka Boom!

The blow back uses the mass of the breechblock, in other words, it is mechanically just possible for the mass of the moving bullet to overcome the inertia of the breechblock and accelerate it to the point that its inertia will continue to operate the breech after the gas pressure drops.

The locking systems just accomplish the same things by mechanical means with the advantage of lighter parts. For example, the mass of a 30-06 breechblock for a blowback rifle would be some 14 pounds, IIRC. A locked breech accomplishes the same thing with a pound of metal.

BigG

April 2, 2008, 09:04 AM

You might want to peruse Hatcher's notebook for some ideas about breeching mechanisms and forces. HTH

Tony Williams

April 2, 2008, 04:08 PM

The term "delayed blowback" tends to cause some confusion as it is used to describe two different systems. The most common is where the breech is never locked but merely retarded in its initial movement. The other is where the breech is locked at the instant of firing, is unlocked (by gas or recoil action) but then operates in the rest of the cycle by blowback. Perhaps the best known example of the latter is the gas-unlocked blowback Hispano 20mm aircraft cannon used by both the RAF and the USAAF/USN in WW2, but some earlier Italian MGs also used this system.

As a result, some gun writers (including myself) prefer to call the first type "retarded blowback" and the second type "delayed blowback", as that describes how each works.

Anyway, all retarded blowbacks use some form of mechanical device to slow the bolt opening, with the exception of a few which use gas tapped from the barrel to hold the breech closed. This was I think first used in some late-WW2 German guns (Volkssturm, or something), but occasionally appears still (e.g. the Steyr GB18 pistol (I hope I'm getting the designations right -I'm away from my sources so running on memory).

In some modern AA cannon (generally Oerlikons) it can be difficult to distinguish between gas-operated and gas-unlocked blowback guns, because although they are gas-operated they fire so fast that the breech opens while the gas pressure is still high, so blowback is an important element of the extraction force - effectively they are hybrid systems.

PercyShelley

April 2, 2008, 06:57 PM

Anyway, all retarded blowbacks use some form of mechanical device to slow the bolt opening, with the exception of a few which use gas tapped from the barrel to hold the breech closed. This was I think first used in some late-WW2 German guns (Volkssturm, or something), but occasionally appears still (e.g. the Steyr GB18 pistol (I hope I'm getting the designations right -I'm away from my sources so running on memory).

Volkssturmgewehr 1-5, and gas-retarded blowback is also used in the HK P7, which was more sucessful than the Steyr GB.

It does allow for low, stationary barrel, so every so often someone tries to revive it:

http://ictechnology.com/home/

BigG

April 3, 2008, 07:40 AM

So, apart from delayed blowback aircraft applications, the small arms are all retarded blowback? (As I always maintained).

Tony Williams

April 3, 2008, 01:07 PM

I think that some of the Fiat/Revelli MGs were delayed blowbacks, but I'd have to check once I got home.

BigG

April 3, 2008, 02:50 PM

Ok, but the point I was going for was they were few and far between. Kind of an evolutionary dead end compared to gas operated and recoil operated which seem to have dozens or hundreds of examples.

The coolest modern application of recoil I've noted is the Benelli-Beretta shotguns with the heavy bolt carrier that actually compresses a heavy spring during recoil and is flung back by same to accomplish the ejection - loading cycle by inertia.

That's an example of someone thinking outside the box. ;)

SDC

April 3, 2008, 06:49 PM

Kind of an evolutionary dead end compared to gas operated and recoil operated which seem to have dozens or hundreds of examples.

True enough, but this is mainly because gas and recoil operation are a lot more reliable and forgiving under most circumstances. The Schwarzlose 1907 heavy machine gun also used a "delay"/"retarded"/"hesitation" mechanism, but the early forms of this MG also needed to oil the cases, otherwise it would pull the heads off of the cases.